Update: pulled the plugs today and they seem fine and it's certainly not running rich. I tried to take a picture but it didn't come out right but the plugs were clean; the electrode was grey with a trace of tan, the white bit was clean and in good order. If you look at this table, it was like the plug in the top picture. (Mine was more grey) Plug condition table

So, given that the carbs are OK and the plugs are OK, what are my options?

Regarding the dyno., the place I took it to race their own bikes and they'd had one of their own bikes on the dyno that morning and that hadn't shown any abnormal readings so I'm assuming the dyno is OK, too.

I'm going to speak to the dyno operator to see what he says but I thought I'd run it by you lot first.

Incidentally, if it was running slightly lean, what would the plugs look like? As I've described mine? If it was running very lean, what would they look like?